(1) Field of the Invention
The present invention relates to a technique for safer operation of a metal halide lamp.
(2) Description of the Related Art
A conventional metal halide lamp, as shown in FIGS. 10A and 10B, has the following structure. An outer tube 102 is sealed at one end, and a base 112 is attached to the other end. The outer tube 102 accommodates an arc tube 105, stem wires 103a and 103b that support the arc tube 105, a glass sleeve 110 that encloses the arc tube 105 and acts to protect against explosions, and plates 108 and 109 that hold respective ends of the sleeve 110.
Nitrogen gas is inserted into the outer tube 102 so as to have a pressure of 100 kPa in operation.
A glass stem 101 is welded at the end of the outer tube 102 that is held by the base 112. The stem 101 supports the two stem wires 103a and 103b that supply current to electrodes.
The arc tube 105 is made up of a cylindrical main tube part that is the central part of the arc tube 105, and two cylindrical, narrow tube parts that are provided on either end of the main tube part. Predetermined amounts of a metal halide, mercury, and a rare gas are sealed in the arc tube 105. The metal halide serves as a light emitting material, the mercury as a buffer, and the rare gas as a starter gas.
A pair of electrodes are provided opposing each other in the main tube part.
An end of each electrode is electrically connected to one end of feeders 104a and 104b, respectively. The feeders 104a and 104b are sealed in the narrow tube parts by glass fritting.
The other end of each of the feeders 104a and 104b extends out of the narrow tube part, and is electrically connected to the stem wires 103a and 103b, respectively.
In order to light the metal halide lamp, a driving circuit that includes an igniter (not illustrated), a ballast (not illustrated), and a power circuit (not illustrated), is usually provided.
During startup, the igniter adds a high voltage pulse to a sine wave voltage that is applied during steady state, thereby causing weak discharge in the vicinity of a starting wire 107 and an electrode 114. Initial electrons discharged here cause arc discharge at a low starting voltage across the pair of electrodes in the arc tube 105, as shown in FIG. 11A.
In this way, startup performance is improved in a conventional metal halide lamp by inclusion of a starting wire.
However, while able to start with a low voltage, the following problems exist in conventional metal halide lamps.
The inner walls of the arc tube 105 are subject to high temperature and high pressure during discharge. As a result, when the metal halide lamp has been used for a substantial length of time, heat fatigue may cause breakage of the arc tube 105, as shown in FIG. 11B.
When the discharge tube 105 breaks, the rare gas, mercury and metal halide escape. Consequently, arc discharge ceases, and the current value drops to 0.
At this time, the igniter detects that the lamp voltage has risen, and adds a high voltage pulse to the sine wave voltage, in the same manner as at startup.
This causes breakage of insulation between one of the electrodes and the part of the starting wire 107 whose distance (rb) from the electrode is shorter than the distance (ra) between the electrodes, and subsequently causes arc discharge, in other words abnormal discharge across the electrode and the starting wire 107.
Note that this abnormal discharge is also called outer tube discharge.
The starting wire 107 is made of a narrow molybdenum wire, or the like, and therefore when abnormal discharge occurs, a C part where the discharge starts (shown in FIG. 11B) melts. However, abnormal discharge continues because a portion of the starting wire that is above the melted C part is connected to the electrode 113.
Melting consequently progresses, and, as shown in FIG. 11C, while the portion of the starting wire above the C part continues to melt, the discharge distance (rc) increases, extending to a D part.
As the discharge distance reaches the discharge distance (rc), the voltage necessary to continue abnormal discharge can no longer be provided, and the abnormal discharge ceases.
During the progression to this point, breakage of the ballast and the like often occurs due to the large current that accompanies the abnormal discharge. Furthermore, there is also a possibility of cracking and breakage of the outer tube 102 as a result of the temperature increase caused by the abnormal discharge.